Nanoscale manganese oxide on ultraporous carbon nanoarchitecture

ABSTRACT

A composite concerning a porous carbon structure having a surface and pores and a coating of MnO 2  on the carbon surface, wherein the coating does not completely fill or obstruct a majority of the pores, and wherein the coating is formed by self-limiting electroless deposition.

This application claims the benefit of provisional application No. 60/847,399 filed on Sep. 11, 2006.

Electrochemical capacitors (also denoted as supercapacitors or ultracapacitors) are a class of energy-storage materials that offer significant promise in bridging the performance gap between the high energy density of batteries and the high power density derived from dielectric capacitors. Energy storage in an electrochemical capacitor is accomplished by two principal mechanisms: double-layer capacitance and pseudocapacitance.

Double-layer capacitance arises from the separation of charge that occurs at an electrified interface. With this mechanism the capacitance is related to the active electrode surface area, with practical capacitances in liquid electrolytes of 10-40 μF cm⁻². Electrochemical capacitors based on double-layer capacitance are typically designed with high-surface-area carbon electrodes, including carbon aerogels, foams, nanotube/nanofiber assemblies, and papers. Carbon aerogels and related porous carbons are particularly attractive due to their high surface areas, high porosities, and excellent conductivities (>40 S cm⁻¹). Although the high-quality porosity of such carbon nanoarchitectures supports rapid charge-discharge operation, the overall energy-storage capacities of carbon electrodes are ultimately limited by their reliance on the double-layer capacitance mechanism.

Pseudocapacitance broadly describes faradaic reactions whose current-voltage profiles mimic those of double-layer capacitors. Because this mechanism involves true electron-transfer reactions and is not strictly limited to the electrode surface, materials exhibiting pseudocapacitance often have higher energy densities relative to double-layer capacitors. The two main classes of materials being researched for their pseudocapacitance are transition metal oxides and conducting polymers.

At present, some of the best candidates for electrochemical capacitors are based on nanoscale forms of mixed ion-electron conducting metal oxides, such as RuO₂, which store charge via a cation-electron insertion mechanism. Ru^(IV)O₂ +xe ⁻ +xH⁺

H_(x)Ru^(III) _(x)Ru^(IV) _(1-x)O₂  [1] Electrodes based on disordered, hydrous RuO₂ yield specific capacitances up to 720 F g⁻¹. However, the application of RuO₂ is limited by the high costs of the ruthenium precursors.

Manganese oxides have recently gained attention as active materials for electrochemical capacitors, primarily due to their significantly lower cost relative to hydrous RuO₂. A survey of recent publications, combined with research findings at the NRL, shows that when prepared in traditional electrode configurations, such as micron-thick films or in composite electrodes containing carbon and binders, MnO₂ delivers a specific capacitance of ˜200 F g⁻¹, which is competitive with carbon supercapacitors, but far short of the 720 F g⁻¹ obtained with hydrous RuO₂. However, as reported independently by Pang et al. and Toupin et al., when MnO₂ is produced as a very thin film (tens of nanometers or less) on a planar current collector, specific capacitances of 700 F g⁻¹ and 1380 F g⁻¹ can be achieved, respectively. This disparity in measured capacitance can be attributed to poor long-range electronic and/or ionic conductivity for MnO₂, which can inhibit the charge-discharge process in conventional electrode designs. Although ultrathin films of MnO₂ deliver high specific capacitance, this configuration can limit the area-normalized capacitance for practical EC devices.

Alternatives involve electrode structures incorporating carbon nanotubes (an expensive carbon substrate) or alternative MnO₂ deposition methods (e.g., electrodeposition, sputtering), which are more complicated, costly and more difficult to control.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic of a hybrid electrode structure comprising a highly porous carbon nanostructure coated with nanoscopic MnO₂ deposits.

FIG. 2 shows a schematic of electrodeposition on an ultraporous electrode structure in (i) a poorly controlled manner in which the pores are ultimately blocked by the growing film and (ii) a controlled, self-limiting deposition.

FIG. 3 shows scanning electron micrographs of (a and b) 4-h acid-deposited MnO₂-carbon, (c and d) 4-h neutral-deposited MnO₂-carbon, and (e and f) bare carbon nanofoam.

FIG. 4 shows a scanning electron micrograph (top) for the cross-section of a 4-h neutral-deposited MnO₂-carbon, and corresponding elemental mapping images (middle and bottom) of this same area of the sample for carbon and manganese content, respectively.

FIG. 5 shows cyclic voltammograms at 2 and 20 mV s⁻¹ in 1 M Na₂SO₄ for bare carbon nanofoam (-) and 4-h MnO₂-carbon nanofoam deposited from acidic permanganate solution ( - - - ) and neutral permanganate solution ( . . . ).

FIG. 6 shows a Nyquist plot and a capacitance vs. frequency profile for acid-deposited, neutral-deposited MnO₂-carbon nanofoam and bare carbon nanofoam.

FIG. 7 shows scanning electron micrographs of the surfaces of 4-h neutral-deposited MnO₂-carbon nanofoams, where AgMnO₄ is the deposition precursor, both with (top) and without (bottom) the addition of a pH 6.9 phosphate buffer.

DESCRIPTION

Nanostructured MnO₂-carbon nanoarchitecture hybrids can be designed as electrode structures for high-energy-density electrochemical capacitors that retain high power density. Homogeneous, ultrathin coatings of nanoscale MnO₂ can be incorporated within porous, high-surface-area carbon substrates (such as carbon nanofoams) via electroless deposition from aqueous permanganate under controlled pH conditions. The resulting hybrid structures exhibit enhanced gravimetric, volumetric, and area-normalized capacitance when electrochemically cycled in aqueous electrolytes. This design can be extended to other mesoporous and macroporous carbon forms possessing a continuous pore network.

The performance limitations of MnO₂ for electrochemical capacitors can be addressed with a hybrid electrode design, by incorporating discrete nanoscale coatings or deposits of MnO₂ onto porous, high-surface-area carbon structures (see FIG. 1). In such a configuration, long-range electronic conduction is facilitated through the carbon backbone and solid-state transport distances for ions through the MnO₂ phase can be minimized by maintaining a nanoscopic carbon ∥MnO₂∥ electrolyte interface throughout the macroscopic porous electrode. Such a design can be realized using various types of porous carbon substrates including but not limited to aerogels/nanofoams, templated mesoporous carbon, and nanotube/nanofiber assemblies.

The synthesis and electrochemical characterization of MnO₂-carbon composites has been reported and primarily focused on incorporating nanoscale MnO₂ deposits onto carbon nanotubes using a variety of approaches including simple physical mixing of the components, chemical deposition using such precursors as KMnO₄, and electrochemical deposition. In these cases, the incorporation of MnO₂ improves the capacitance of the electrode structures that contain the MnO₂-modified carbon nanotubes; however, the overall specific capacitance for the composite structures is typically limited to <200 F g⁻¹, even for electrodes with high weight loadings of MnO₂. One exception was reported by Lee et al., who demonstrated specific capacitances of up to 415 F g⁻¹ as normalized to the MnO₂ of the composite structure. However, those results were achieved only for micron-thick electrode structures containing MnO₂-modified carbon nanotubes, again a configuration that limits energy density.

Templated mesoporous carbon powders have also been used as a substrate for MnO₂ deposition as demonstrated by Dong et al., who used the reaction of permanganate with the carbon substrate to generate nanoscale MnO₂ deposits directly on the mesopore walls. Electrochemical testing of the resulting MnO₂-mesoporous carbon structures revealed that the MnO₂ deposits themselves exhibited a specific capacitance of ˜600 F g⁻¹, which approaches the 700 F g⁻¹ reported by Pang et al. for nanometers-thick MnO₂ films. Despite the high MnO₂-normalized capacitance, the overall specific capacitance of the hybrid MnO₂-mesoporous carbon structure was limited to 200 F g⁻¹, due to the relatively low weight loading (up to 26%) of MnO₂. The extent of MnO₂ deposition within the mesoporous carbon substrate can be limited by the inherently small pore size (˜3 nm) of the carbon.

The investigations of Dong et al. and Lee et al. demonstrate that nanoscopic deposits of MnO₂ on high-surface-area substrates do deliver high specific capacitance. To further optimize the performance of MnO₂-carbon hybrid structures for electrochemical capacitor applications, at least three design parameters must be addressed: (i) achieving high weight loadings of MnO₂ (>50 wt. %); (ii) fabricating electrode structures with macroscopic thickness (tens to hundreds of microns); and (iii) retaining a through-connected pore network in 3D and with pores sized at >5 nm.

The use of thick carbon substrates, as opposed to dispersed carbon powders, presents new challenges for achieving homogeneous MnO₂ deposition throughout the electrode structure, while preserving the native pore structure of the carbon template. A high-quality pore structure is vital for high-rate EC operation, facilitating electrolyte infiltration and ion transport. These properties can be achieved by using coating methods that are inherently self-limiting as shown schematically in FIG. 2. Described in this disclosure is the self-limiting electroless deposition of nanoscale MnO₂, based on the redox reaction of aqueous permanganate (MnO₄ ⁻) and carbon aerogel/nanofoam substrates. The MnO₂ prepared by this protocol is a complex structure incorporating cations and water; this material will be designated as MnO₂ in the body of this application.

EXAMPLE 1 Electroless Deposition of MnO₂ on Carbon Nanofoams

Carbon nanofoam papers were either purchased from a commercial source or prepared in-house. MnO₂-carbon nanoarchitecture hybrids were created based on the reductive decomposition of permanganate from aqueous solutions. The carbon nanoarchitecture surface can serve as a sacrificial reductant, converting the aqueous permanganate to insoluble MnO₂.

Carbon nanofoam substrates (˜170 μm thick) were first wetted in an aqueous solution of controlled pH (0.1 M H₂SO₄, 0.1 M Na₂SO₄, or 0.1 M NaOH) by vacuum infiltration. The samples were then soaked in 0.1 M NaMnO₄ at each respective pH for a period of 5 min to 4 h. The MnO₂-carbon nanofoam papers were rinsed thoroughly with ultrapure water and subsequently dried at ˜50° C. under N₂ for 8 hours and then under vacuum overnight.

Control of the permanganate reduction reaction can be required to achieve nanoscale MnO₂ deposits throughout the carbon nanoarchitecture as well as to inhibit the formation of thick MnO₂ coatings on the outer boundary of the carbon electrode. Preliminary results suggest that pH can be a critical factor in determining the quality of the MnO₂ deposition.

As shown by the scanning electron micrographs (SEM) in FIGS. 3 a and 3 b, under acidic conditions, permanganate reacts with carbon nanofoams to primarily form thick crusts of MnO₂ on the outer boundary of the carbon electrode, presumably due to the autocatalytic decomposition of permanganate in acid. A cross-sectional image of the acid-deposited MnO₂ crust, shown in the inset of FIG. 3 a, reveals that the crust thickness was ˜4 μm for a 4-h deposition. By contrast, permanganate reduction in neutral or basic pH solutions results in homogeneous MnO₂ deposits (neutral sample, FIGS. 3 c and 3 d) that are nearly indistinguishable from the bare carbon aerogel (FIGS. 3 e and 3 f) with no MnO₂ crust formation at the outer boundary of the nanofoam electrode.

The MnO₂ mass uptake (up to ˜60% for a 24-h deposition) can be relatively independent of the solution pH. The SEM analysis further confirmed that the porous texture of the initial carbon nanofoam can be largely retained following MnO₂ deposition (see FIGS. 3 d and 3 f). The retention of the nanofoam's high-quality pore structure can result in better electrochemical performance under high-rate charge-discharge operation.

The cross-sectional SEM and elemental mapping images of the MnO₂-carbon nanofoam synthesized under neutral conditions in FIG. 4 show that the Mn can be evenly distributed throughout the thickness of the electrode structure. Incorporation of the MnO₂ domains within the porous carbon nanoarchitectures in such a homogeneous, conformal fashion can result in hybrid electrode structures with superior performance relative to the less ideal structures obtained under acidic deposition conditions. X-ray photoelectron spectroscopy was used to verify that Mn deposits were primarily in the form of Mn^(III/IV)O₂, with no residual MnO₄ ⁻.

EXAMPLE 2 Electrochemical Characterization of Hybrid Structures

The MnO₂-carbon nanofoam electrodes were wetted with 1 M Na₂SO₄ under vacuum for electrochemical analysis and characterized in a conventional three-electrode electrochemical cell using techniques such as cyclic voltammetry, impedance spectroscopy, and galvanostatic charge-discharge measurements. Representative cyclic voltammograms of the bare carbon aerogel, 4-h acid-deposited, and 4-h neutral-deposited MnO₂-carbon nanofoam electrodes in 1 M Na₂SO₄ at 2 and 20 mV s⁻¹ are presented in FIG. 5.

A saturated calomel reference electrode (SCE) and reticulated vitreous carbon auxiliary electrode were used in all electrochemical measurements. At 2 mV s⁻¹, all curves exhibit a nearly symmetrical rectangular shape, indicative of relatively low uncompensated electrode or solution resistance. The gravimetric (normalized to total sample mass), volumetric, and area-normalized capacitance values calculated from these curves between 0.1 and 0.6 V vs. SCE are presented in Table 1. Both the total gravimetric and volumetric capacitance values increase for the acid and neutral-deposited samples. Notably, the gravimetric capacitance increases by a factor of 2 for the neutral-deposited sample, while the volumetric capacitance is over 4 times greater. It is important to note that in the case of homogeneous, nanoscopic MnO₂ deposits like those in the neutral-deposited hybrid electrode, the incorporation of MnO₂ can contribute additional capacitance without increasing the bulk volume of the electrode structure.

When pulse power is required in a footprint- or area-limited configuration, as in microelectromechanical (MEMS) based and on-chip devices, the area-normalized energy-storage capacity should also be considered. Although the area-normalized capacitance is often not reported for MnO₂/carbon composites, it is usually around 10-50 mF cm⁻². In contrast, the present hybrid electrode design maintains the advantages of a nanoscopic electrode/electrolyte interface while projecting the electrode structure in three dimensions with a limited footprint, such that the area-normalized capacitance for the neutral-deposited MnO₂-carbon hybrid electrodes is orders of magnitude greater at ≧2 F cm⁻².

The upper and lower limits of capacitance attributed to MnO₂ in Table 1 were estimated using one of two assumptions: (1) all capacitance arises from the MnO₂ phase (upper limit) or (2) the total sample capacitance was the sum of the carbon double-layer capacitance and the MnO₂ capacitance (lower limit). Although the capacitance attributable to the MnO₂ phase for the acid- and neutral-deposited samples likely falls within this range, the capacitance contribution from the carbon is expected to be different for the acid and neutral case because of the variation in the MnO₂ spatial distribution. For example in the acid case, the double-layer capacitance contribution of carbon should be largely unaffected due to the limited MnO₂ deposition in the electrode interior. Thus, the MnO₂-normalized capacitance is likely near the lower estimated limit, while that for the neutral sample is expected to be higher as a result of extensive MnO₂ coating the carbon on the electrode interior.

Although the total capacitance enhancement for the acid and neutral-deposited MnO₂ samples presented in Table 1 is similar, the difference in the spatial distribution of MnO₂ for the two samples results in a sloping voltammetric curve for the boundary-crusted, acid-deposited MnO₂-carbon nanofoam at 20 mV s⁻¹ due to increasing resistance that results from non-uniform MnO₂ deposition. This increased resistance is confirmed by electrochemical impedance analysis (E_(DC)=200 mV vs. SCE) presented in FIG. 6 (similar results were observed at 0 and 600 mV).

At high frequencies, the uncompensated solution resistance (R_(Ω)) of each electrode is similar, as shown in the Nyquist plot (FIG. 6 a). However, the large hemispherical component for the MnO₂-carbon nanofoam electrode deposited under acidic conditions is indicative of polarization as expressed by a charge-transfer resistance (R_(p)) of about 15Ω. In contrast, the profile for the neutral-deposited sample is more similar to that of the bare carbon nanofoam, with an R_(p) of ˜1Ω. The capacitance vs. frequency profile of the neutral-deposited sample in FIG. 6 b shows that from about 0.01 to 1 Hz, the MnO₂ component can increase the capacitance of the bare carbon nanofoam. As the frequency increases, the capacitance for both electrodes begins to decrease, falling to below 1 F g⁻¹ around 200 Hz. The initial capacitance increase for the acid-deposited sample at 0.01 Hz, with respect to the bare nanofoam, can be much lower than that for the neutral-deposited sample and begins to decrease between 0.1 and 1 Hz, falling below 1 F g⁻¹ at 30 Hz.

The higher resistance and lower capacitance for the acid-deposited sample is likely due to the thick MnO₂ crust that forms on the electrode exterior, hindering electron and ion transport, while the more ideal homogeneous distribution of MnO₂ in the sample deposited under neutral conditions results in electrochemical characteristics more similar to the bare nanofoam.

TABLE 1 Specific MnO₂-specific Volumetric Area-normalized capacitance capacitance range capacitance capacitance (F g⁻¹ _(C+MnO) ₂ ) (F g⁻¹ _(MnO) ₂ ) (F cm⁻³) (F cm⁻²)** Bare nanofoam* 53 — 20  0.56 Acid-deposited* 92 150-220 81 1.4 Neutral-deposited* 110  170-230 90 1.5 *These data are derived for 4-h depositions from acidic or neutral permanganate solutions. **Normalized to the geometric area of one face of the nanofoam electrode.

The electroless deposition described herein can be a simple, cost-effective, and scaleable approach for synthesizing MnO₂-carbon hybrid nanoarchitectures with electrochemical capacitance that is superior to unmodified carbon substrates. This disclosure demonstrates that by controlling solution pH during the deposition process, homogeneous MnO₂ deposits are achieved throughout macroscopically thick porous carbon templates.

There can be many benefits of homogenous MnO₂ deposition as can be evident when such structures are electrochemically analyzed. For example, MnO₂-carbon hybrids that exhibit uniform MnO₂ distribution (neutral-pH deposition) also exhibit higher overall gravimetric and volumetric capacitance, and higher MnO₂-specific capacitance than acid-deposited MnO₂-carbon hybrids, in which the MnO₂ is primarily deposited as a crust on the outer boundaries of the electrode.

Uniform deposition within the interior of the carbon nanoarchitecture also can result in greater enhancement when the volumetric capacitance is considered, as the addition of the MnO₂ component contributes additional capacitance without increasing the bulk volume of the electrode structure. For example, with a carbon nanofoam coated under neutral-pH conditions the gravimetric capacitance is increased by a factor of 3.3, while the volumetric capacitance is increased by a factor of 4.1. Even greater enhancements in electrochemical performance for these hybrids can be realized with further optimization of the electroless deposition conditions (e.g., varying the solution temperature, precursor concentration, permanganate counterion—including transition metal speciation, constituents that define the acidic or neutral medium including buffers) and also by varying the carbon template pore structure, particularly targeting larger pore sizes (100-200 nm) and higher overall porosity, which should result in higher mass loadings of MnO₂.

In an example of two modifications to the deposition protocol, carbon nanofoams were soaked in nominally neutral aqueous solutions of commercially available AgMnO₄ (substituted for NaMnO₄) under buffered and unbuffered conditions. The morphology of the resulting Ag_(x)Mn^(III/IV) _(1-x)O₂ deposits is affected by the presence or absence of buffering agents. Without buffer, the oxide coating is more nodular and preferentially deposited on the outer boundary, while from buffered medium, the deposit is more uniformly distributed, not nodular, and less thick (as seen in FIG. 7).

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any reference to claim elements in the singular, e.g., using the articles “a,” “an,” “the,” or “said” is not construed as limiting the element to the singular. 

1. A composite comprising a porous carbon structure comprising a surface and pores wherein the pores have an average diameter of from about 2 nm to about 1 μm; and a coating of MnO₂ on the carbon surface; wherein the coating does not completely fill or obstruct a majority of the pores; wherein the coating is formed by self-limiting electroless deposition; wherein the porous carbon structure is a carbon aerogel; and wherein cations and water are incorporated within the MnO₂.
 2. The composite of claim 1, wherein the coating has a thickness of less than about 50 nm.
 3. The composite of claim 1, wherein the coating has a thickness of less than about 10 nm.
 4. A capacitor comprising an anode, a cathode, and an electrolyte, wherein the anode, the cathode, or both comprise: a composite comprising a porous carbon structure comprising a surface and pores; and a coating on the surface comprising MnO₂ wherein cations and water are incorporated within the MnO₂; wherein the coating does not completely fill or obstruct a majority of the pores; wherein the porous carbon structure is a carbon aerogel; and wherein the pores have an average diameter that ranges from about 2 nm to about 1 μm; and a current collector in electrical contact with the composite.
 5. The capacitor of claim 4, wherein the coating has a thickness of less than about 50 nm.
 6. The capacitor of claim 4, wherein the coating has a thickness of less than about 10 nm.
 7. The capacitor of claim 4, wherein the electrolyte comprises aqueous sodium sulfate.
 8. The capacitor of claim 4, wherein the electrolyte comprises a liquid comprising an aqueous, nominally neutral (pH 6-8) electrolyte with or without buffering components.
 9. The capacitor of claim 4, wherein the electrolyte comprises a liquid comprising an aqueous, basic (pH>8) electrolyte with or without buffering components.
 10. The capacitor of claim 4, wherein the electrolyte comprises a liquid comprising a nonaqueous liquid of sufficient dielectric constant to dissociate salts soluble in the nonaqueous liquid.
 11. The capacitor of claim 4, wherein the coating is formed by self-limiting electroless deposition.
 12. A capacitor comprising an anode, a cathode, and an electrolyte, wherein the anode, the cathode, or both comprise: a composite comprising a nanostructured porous carbon structure comprising a surface and interpenetrating continuous network of pores; and a coating on the surface comprising MnO₂ wherein the MnO₂ coating is homogenous and evenly distributed throughout and has a thickness of less than about 10 nm; wherein the coating does not completely fill or obstruct a majority of the pores; wherein the porous carbon structure is a carbon aerogel; wherein the pores have an average diameter that ranges from about 2 nm to about 1 μm; wherein the interpenetrating continuous network of pores is retained; and a current collector in electrical contact with the composite. 